Lignocellulosic biomass is an abundant and renewable raw material for biofuel production. However, the initial conversion of insoluble lignocellulosic biomass into cell-permeable and readily fermentable sugars presents a significant technical challenge and major bottleneck in the biofuel production process. Improved means to overcome this bottleneck are therefore needed to unlock the full potential of lignocellulosic biomass as a versatile energy source.
The natural degradation of biomass is achieved by fungal microorganisms through their secretion of lignocellolytic enzymes. For example, the filamentous fungus and laboratory model organism Neurospora crassa (N. crassa) is often found in the wild growing upon recently burnt plant matter, where it secretes cellulases and thereby initiates the depolymerization of plant cell walls. Based on their natural role in lignocellulose degradation, filamentous fungi and their lignocellolytic enzymes have great potential as catalysts of biomass degradation in biotechnological production processes.
However, whereas cellulase secretion in filamentous fungi is effectively induced by insoluble plant cell wall components, such as cellulose, hemicellulose, or xylan, soluble inducers are much less effective. For example, cellobiose, the main soluble end product of cellulases, induces cellulases in several species of filamentous fungi, including Hypocrea jecorina (Trichoderma reesei; T. reesei) and Aspergillus species (A. niger, A. nidulans, A. oryzae) but at much lower levels than cellulose itself. However, one problem with insoluble inducers is that cellulase can adhere to insoluble inducers, resulting in reduced yields of secreted enzyme activity.
The processing of insoluble biomass matter is a heterogeneous process and access to biomass surfaces is limiting for fungal cells. In rich fungal cultures therefore, a large population of cells will be free-floating and not secreting high levels of active cellulase enzymes, due to their lack of contact with inducing plant surfaces. To optimize the production of proteins, including cellulase enzymes, in such cell suspensions and thereby facilitate biomass degradation, cellular systems are needed that secrete high levels of active proteins after induction with soluble small molecules, such as cellodextrin.